FIG. 1 schematically shows a prior art isolated switching converter 10 comprising a primary side circuit 101 and a secondary side circuit 103 coupled via a couple device 102, e.g., a transformer. The isolated switching converter 10 receives an input voltage Vin by the primary side circuit 101 and provides an output Voltage Vout by the secondary side circuit 103. However, information of the output voltage Vout needs to be fed back from the secondary side circuit 103 to the primary side circuit 101 to control the power conversion. In the prior art isolated switching converter 10 in FIG. 1, an opto-coupler 106 is adopted to feedback the output voltage and to realize electrical isolation between the primary side circuit 101 and the secondary side circuit 103 to protect the user on the secondary side circuit 103 from relatively high voltages on the primary side circuit 101 (such as a high input voltage Vin). The output voltage Vout of the switching converter 10 is provided to a photosensitive diode D0 of the opto-coupler 106 via a secondary feedback circuit 104, and is then further provided to a photosensitive transistor T0 of the opto-coupler 106 in form of an optical current lopto, wherein the optical current lopto is provided to the primary side circuit 101 to control the power conversion. TL431 shown in FIG. 1 is a three-terminal adjustable shunt regulator produced by Texas Instruments Inc. (TI), and is widely adopted in the feedback circuits of the isolated switching converters.
FIG. 2 shows a prior art relation curve that the optical current lopto varies with an output power Pout of an isolated switching converter. As can be seen from FIG. 2, the lower the output power Pout, the larger the optical current lopto, which means that the optical current lopto is relatively large under light load or no load condition, and reaches to the maximum when the output power Pout reaches its minimum. This characteristic seriously hurts the light/no load efficiency of the isolated switching converter.
So a circuit to improve light/no load efficiency by decreasing the optical current lopto under light/no load condition is necessary.